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European
Sealing
Association e.V.
Tightness in gasketed
flanged unions
PART II: New Standards for Calculation and Testing.
The early attempts in the USA and Germany
By Francisco J. Montero, ESA member
6.- New pressure equipment
calculation standards and new
gasket testing methods
To summarize Part I, new materials and
environmental concerns brought forward
the need to introduce new variables in
the design, calculation, operation and
maintenance of industrial pressure systems.
The first attempts were made in the USA
and Germany. Additionally, as we shall
see in Part III, Europe must harmonize
the applicable legislation and standards
to ensure single market conditions as
required by the Treaty of Rome.
6.1.- USA
The search for “better” m and y factors
started in 1974 at the Pressure Vessel
Research Council (PVRC) on the initiative
of the ASME Boiler and Pressure Vessel
Code and ASTM committees. This goal
was found too simplistic. Research was
needed on gasket properties behind
quantitative leak levels. In 1984, the PVRC
widened its remit to include concerns
about high temperature reliability of the
new asbestos-free materials coming to
the market. A new testing programme
was launched at the École Politechnique
de Montréal under the auspices of PVRC
and the Material Testing Institute for
the Chemical Process Industries (MTI).
The main new test procedure was
ROTT (Room Temperature Operational
Tightness Test), designed to obtain
the gasket parameters needed for the
flanged connection calculation. A new
dimensionless number, Tp (Tightness
parameter), was introduced to link the
tightness level of a connection with the
internal pressure. On a given connection,
for each gasket stress there is a Tp value.
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The higher the Tp value, the higher the
tightness of the connection.
The ROTT test is a combined compression,
relaxation and helium leakage test at
room temperature using 4” NB flanges.
It consists of two parts. The first refers
to the assembly conditions. Growing
stresses are applied on the gasket and
leakage measurements are taken with
different inner helium pressure levels. The
second part represents the changes on
gasket stresses in service (unloading from
hydraulic fluid pressure, external loads,
etc.) by applying successive loading and
unloading cycles and measuring the changes
in leak rates at a constant inner pressure.
Representing, in ordinates, the logarithm
of the loading and unloading gasket stress
and in abscissae, the logarithm of Tp, a
graph (Fig.1) is obtained with three new
gasket factors, Gb, a, Gs.
Gb, is the loading stress corresponding
to Tp=1, (log Tp=0).
a, is the slope of the gasket loading
line. Combined values of Gb and a
describe the seating characteristic of
a gasket and its capacity to develop
tightness upon initial sealing.
Fig.1 Log-Log gasket stress versus tightness
parameter.
Gs, is a theoretical point, representing
the intercept of the set of possible
unloading lines with the Tp=1
line. It represents the gasket
potential to maintain tightness after
pressurization and during operation.
The PVRC drafted a new version of the
calculation method, (Non-mandatory
Appendix BFJ), incorporating the following
aspects:
•
•
•
•
•
Introduction of the Tightness Class
concept as a design criterion defining
higher or lower tightness levels
Introduction of the tightness-linked
gasket parameters
Introduction of a flange rigidity index
Consideration of forces and moments
external to the connection
Consideration of the loads on the
gasket’s bars at heat exchangers
divisions.
The PVRC presented the new calculation
method, with these new factors, as an
alternative method to the traditional and
still valid ASME Code, Section VIII, Div 2,
Appendix 2. The ROTT test itself gives
interesting gasket data. The tests will be
used later as the basis of the new DIN and
EN standards, very similar to the ROTT
test, but not to the subsequent PVRC
graphic elaboration of the data. This graphic
stage of the procedure is contradictory to
results obtained in practice. The unproven
hypothesis of a common intersection point
with the Tp=1 axis for all the possible
unloading lines has been proven wrong
by experimental data. Generation of the
three gasket factors, Gb, a, Gs, is thus not
reliable and Non-mandatory Appendix
BFJ remains only an alternative to the
traditional calculation.
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163
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6.2.- Germany
A tentative first attempt was made in
1986 with standard DIN V 2505, revised
into DIN 2505 E 4/90. This new calculation
method did not use tightness-linked gasket
parameters and was superseded in 2001
when EN 1591-1 was approved.
In September 1995 the new DIN 28090
and DIN 28091 standards were approved
and published to become the German
reference for the asbestos-free sheet
gasket materials, addressing fully the
tightness quantification concerns. Based
however in a small experimental basis,
these standards have seen most of their
content replaced by the new European EN
standards. Their main traits were as follows:
-
DIN 28090 and DIN 28091 were
conceived as complementary. The
first refers to test methods and the
resulting gasket parameters. The
second refers to technical conditions
-
-
of supply. The respective contents are
summarized below.
DIN 28090, Part 1, defines the test
methods required to complete the
qualification programme of all static
gaskets and obtain the factors or
parameters needed for the calculation
of flanged connections. The tests
themselves are quite similar to the
North-American ROTT tests and are
shown on the tables below:
DIN 28090, Part 2, contains the test
methods to be applied as part of
the manufacturer’s quality control
procedures. The requirements for the
different materials are defined by DIN
28091.
DIN 28090, Part 3, defines the tests
for chemical resistance of the gasket
materials. The main test is made with
a flanged test rig containing the
chemical fluid. The weight and
compressive load on the rig are
recorded and the rig is heated in an
oven to the desired temperature.
The weight loss and the fasteners
relaxation are measured at frequent
intervals during 1000 hours.
DIN 28091-1 defines general
requirements regarding dimensions and
markings. Technical delivery conditions
are defined for compressed fibre
materials (DIN 28091-2), PTFE gasket
materials (DIN 28091-3) and graphite
gasket materials (DIN 28091-4).
Leakage tests to determine the following parameters:
Leak rate λ, for sealability
classes:
Compressive stresses on gasket
L1,0≤ 1,0 mg.s-1m-1
σVU/L, minimum initial stress
L0,1≤ 0,1 mg.s-1m-1
σBU/L, minimum service compressive stress
L0,01≤ 0,01 mg.s-1m-1
Compression and creep tests to determine the following parameters:
Compression Test
Creep Test
σVO maximum initial compressive
stress
∆hD compression set under constant load
σBO maximum service compressive
stress
σBO maximum service compressive stress
ED elastic modulus
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November 2012
In December 2004 EN 13555 became
effective as the European Standard and
DIN 28090, Part 1, was automatically
superseded. However DIN 28090-2 and
DIN 28091 are still in force in Germany
and its area of influence.
6.3.– European Union. EN
Standards
In April 2001 the new EN-1591-1
standard was approved, within the
framework of Directive 97/23/CE PED
which requires the harmonization of
everything related to pressure equipment.
Its title: “Flanges and their joints –
Design rules for gasketed circular flange
connections – Part 1: Calculation method”.
These rules are the basis for Annex G
of EN 13445-3, as an alternative to the
Chapter 11 method of the same standard.
In 2009 a new version of EN 1591-1
was approved. We thus reach the new
European standardization system that will
be described on Part III of our series.
The European Sealing Association
(ESA) has produced this article
as a guide towards Best Available
Techniques for sealing systems and
devices. These articles are published
on a regular basis, as part of their
commitment to users, contractors
and OEM’s, to help to find the
best solutions for sealing challenges
and to achieve maximum, safe
performance during the lifetime
of the seal. The ESA is the voice of
the fluid sealing industry in Europe,
collaborating closely with the Fluid
Sealing Association (FSA) of the USA.
Together, they form a key source
of technical information on sealing
technology, which is the basis for these
articles. For more information, please
visit www.europeansealing.com
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